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The original mammals evolved from reptiles. These small, insect eating, shrew-like creatures, had the advantage of being endothermic - they produced their own heat internally by consuming great amounts of food. This gave them the advantage of being active at night, when reptiles couldn't compete because of the cold. This enabled the mammals to become dominant at the end of the Mesozoic, when the climates got colder and many of the larger reptiles died out.

The problem of keeping the young warm resulted in other adaptations, for example the MARSUPIALS (kangeroos, wombats, wallabies, opossums) - which nurture their young in a pouch; and the PLACENTAL MAMMALS - which carry the young inside the body for a longer period of development, feeding them nutrients from a placenta.

Harry Williams, Historical Geology

Apart from evolutionary changes designed to keep mammals warm; other changes also occurred which were adaptations to the changing environment. Perhaps the most important of these changes was the appearance of GRASSES and vast PRAIRIES in the Miocene. This led to the evolution of grazing mammals - the UNGULATES.

Harry Williams, Historical Geology

Ungulates are characterized by continuously-growing cheek teeth for chewing grass and multi-chambered stomachs for digesting tough grass. Due to the lack of cover on the open prairies, these mammals also developed speed to escape predators - long legs for running and running on toes, which became HOOVES. The ungulates are classified according to the number of toes; PERISSODACTYLS = odd-toed (if middle toe carried weight-> single hoof). Examples: zebra, horse, rhino, tapir.

Particularly important, of course, was the evolution of the order PRIMATES, distinguished by stereoscopic vision and grasping hands - both of which probably evolved to facilitate a life in the trees. The primates evolved into the Late Cenozoic ancestors of Homo sapiens, who appeared in the Pleistocene (more next class).